The image forming principle of liquid crystal display apparatuses requires a polarizing plate be placed on a liquid crystal panel surface. The function of a polarizing plate is to absorb one of polarized components (P-polarized wave and S-polarized wave) that are orthogonal to each other and transmit the other.
Heretofore, dichroic polarizing plates formed of films containing iodine-based or dye-based polymer organic substances have been widely used as the above-described polarizing plate. A typical manufacturing method therefor involves dyeing a polyvinyl alcohol film with a dichroic material such as iodine, performing crosslinking using a crosslinking agent, and uniaxially stretching the resulting product. Since manufacturing of dichroic polarizing plates involves stretching as such, dichroic polarizing plates are usually readily shrinkable. A polyvinyl alcohol film is readily deformable particularly under humid conditions since a hydrophilic polymer is used. Basically, because a film is used, mechanical strength of a device is low and it may be necessary to bond a transparent protective film thereto.
In recent years, usage of liquid crystal display apparatuses has expanded and functions of liquid crystal display apparatuses have become more sophisticated. Under such a trend, individual devices that constitute a liquid crystal display apparatus are required to achieve high reliability and durability. For example, a polarizing plate of a liquid crystal display apparatus, such as a transmission-type liquid crystal projector, that uses a light source with high light intensity receives intense radiation. Thus polarizing plates for use in such apparatuses need to have excellent heat resistance. However, since film-based polarizers such as the one described above are formed of organic substances, there is naturally a limit to enhancing these properties.
In the United States, Corning Incorporated sells a highly heat resistant inorganic polarizing plate under the trade name of Polarcor. This polarizing plate is configured by dispersing silver fine particles in glass and does not use organic substances such as films. The principle is to utilize plasmon resonance of island-like fine particles. That is, light absorption by surface plasmon resonance induced by light incident on island-like fine particles of a noble metal or a transition metal is utilized where the absorption wavelength is dependent on the shape of the particles and the dielectric constant of the surroundings. When the island-like fine particles have an elliptical shape, the resonance wavelengths in the major axis direction and in the minor axis direction differ from each other, creating a polarization property; specifically, a polarization property is obtained in which a polarized component parallel to the major axis on the long wavelength side is absorbed and a polarized component parallel to the minor axis is transmitted. However, the wavelength range in which Polarcor exhibits a polarization property is a range near an infrared zone and does not cover the visible light range as required by liquid crystal display apparatuses. This is due to the physical properties of silver used in island-like fine particles.
PTL 1 describes a UV polarizing plate that applies the principle described above, which is obtained by precipitating fine particles in glass by thermal reduction, and teaches the use of silver as metal fine particles. Presumably, this polarizing plate utilizes absorption in the minor axis direction unlike Polarcor. As illustrated in FIG. 1, the polarizing plate functions at about 400 nm but the extinction ratio is small and the range that absorption can occur is very narrow; thus, even if the technology of PTL 1 is combined with Polarcor, the resulting polarizing plate will not cover the entire visible light range.
NPL 1 provides theoretical analysis of an inorganic polarizing plate that utilizes plasmon resonance of island-like metal fine particles. According to this document, aluminum fine particles have a resonance wavelength about 200 nm shorter than that of silver fine particles and there is a possibility that a polarizing plate that covers the visible light range can be obtained by using aluminum fine particles.
PTL 2 describes several methods for manufacturing polarizing plates using aluminum fine particles. It is described in PTL 2 that silicate-based glass is not preferable as a substrate since aluminum and glass react with each other, and that calcium-aluminoborate glass is suitable (paragraphs 0010 and 0019). However, silicate glass is widely available as optical glass and highly reliable products can be purchased at low cost. It would be economically disadvantageous it silicate glass were not suitable. PTL 2 also describes a method for forming island-like particles by etching a resist pattern (paragraphs 0037 and 0038). In general, a polarizing plate used in a projector needs to be about several centimeters in size and have a high extinction ratio. Accordingly, for a visible light polarizing plate, the resist pattern size needs to be sufficiently shorter than a visible light wavelength, that is, the resist pattern needs to be several tens of nanometers in size. Moreover, in order to obtain a high extinction ratio, the pattern needs to have a high density. When the polarizing plate is for projector use, the pattern needs to have a large area. However, the method described applies high-density fine pattern formation by lithography, and electron beam lithography, for example, must be used to obtain such a pattern. Electron beam lithography is a method in which individual patterns are drawn by an electron beam, and is unpractical due to its low productivity.
PTL 2 also describes that aluminum is removed by a chlorine plasma; however, when etching is performed as such, chlorides usually deposit on the side walls of an aluminum pattern. Although chlorides can be removed by using a commercially available wet-etching solution (for example, SST-A2 produced by TOKYO OHKA KOGYO CO., LTD.), a chemical that reacts with aluminum chlorides also reacts with aluminum however low the etching rate, and thus it is difficult to obtain a desired pattern shape by the described method.
PTL 2 describes another method that involves depositing aluminum on a patterned photoresist by oblique film formation and removing the photoresist (paragraphs 0045 and 0047). However, according to this method, it is presumably necessary to deposit some aluminum on the substrate surface in order to obtain adhesion between the substrate and aluminum. This means that the shape of the aluminum film is different from the appropriate shape described in paragraph 0015, namely, elongated spheres including prolate spheroids. Moreover, paragraph 0047 describes that an over-deposited portion is removed by performing anisotropic etching perpendicular to the surface. The shape anisotropy of aluminum is extremely important in order for a polarizing plate to function. Accordingly, it is presumably necessary to adjust the amounts of aluminum deposited on the resist portion and the substrate surface so that a desired shape can be obtained by etching; however, paragraph 0047 describes that the size of the aluminum deposits is controlled on the scale smaller than one tenth of a micrometer, namely, 0.05 μm, which is extremely difficult, and whether this process is suitable for a high-productivity manufacturing method is doubtful. Moreover, a polarizing plate is required to have a high transmittance in the transmission axis direction, but a substrate formed of glass inevitably reflects several percent of light at the glass interface, and rarely achieves high transmittance.
PTL 3 describes a planarizing plate obtained by oblique deposition. According to this method, fine columnar structures are manufactured by obliquely depositing transparent and opaque substances relative to the wavelength of the working band to obtain a polarization property. Unlike PTL 1, this method is considered highly productive since fine patterns can be obtained by a simple method. The aspect ratio of the fine columnar structures formed of an opaque substance relative to the working band, and the intervals and straightness of the individual fine columnar structures are important factors for obtaining a good polarization property and should be intentionally controlled also from the viewpoint of reproducibility of properties. However, in order to obtain columnar structures, this method takes advantage of a phenomenon that vapor deposition particles do not deposit on portions shadowed by previously deposited layers of vapor deposition particles; thus, it is difficult to intentionally control the above-described features. The document also describes a method for improvement which involves forming polishing marks on a substrate by conducting rubbing treatment prior to vapor deposition; however, the particle diameter of a vapor-deposited film is generally about several lens of nanometers at maximum and pitches smaller than one tenth of a micrometer need to be intentionally formed by polishing in order to control the anisotropy of such particles. However, a typical polishing sheet or the like can form polishing marks on the scale of one tenth of a micrometer at the smallest, and fine polishing marks such as those described above cannot be easily manufactured. As discussed above, the resonance wavelength of Al fine particles is strongly dependent on the refractive index of the surroundings, and the combination of transparent and opaque substances is important in such a case. However, PTL 3 does not describe a combination for obtaining good polarization property in the visible light range. Furthermore, as in PTL 1, several percent of light is inevitably reflected by the glass interface if glass is used to form a substrate.
NPL 2 describes a polarizing plate, known as Lamipol, for infrared communication. This polarizing plate has an Al/SiO2 laminated structure, and exhibits a very high extinction ratio according to this document. NPL 3 also describes that using Ge instead of Al that contributes to light absorption of Lamipol achieves a high extinction ratio at a wavelength of 1 μm or less. It is also expected that a high extinction ratio can be obtained with Te (tellurium) from FIG. 3 of the same document. While Lamipol is an absorption-type polarizing plate that can achieve a high extinction ratio, the thickness of the laminate constituted by a light-absorbing substance and a light-transmitting substance defines the size of the light-receiving surface and Lamipol is not suitable for projector use that requires a polarizing plate with sides of several centimeters.
PTL 4 describes a polarizing plate in which a wire grid structure and an absorption film are combined. When a metal or semiconductor film is used for the absorption film, optical characteristics of the material strongly affect the properties of the absorption film. It is possible to decrease the reflectance of a particular range by changing the material and the thickness of a dielectric film between the wire grid and the absorption film; however, this cannot be easily achieved throughout a wide wavelength range.
While the band can be expanded by using Ta, Ge, or other element that has high absorption, absorption in the transmission axis direction increases simultaneously, resulting in a decrease in transmittance in the transmission axis direction, which is an important property for polarizing plates.
The issues described above can be addressed by using fine particles in the absorption film. However, the methods so far proposed for directly depositing an absorption film by oblique deposition rely on self-assembly of the absorption film to be deposited by shadowing; thus, the method is affected by the physical properties of the material and roughness of the substrate and it is difficult to control absorption characteristics.